The instruments that are currently available for detecting, sizing or otherwise analyzing aerosol particles with radiation (e.g., electromagnetic radiation) use a design whereby a beam of radiation interacts with particles traveling in a beam of particles so as to cause the particles to emit mass or energy which is then detected and analyzed. The beam of radiation and the beam of particles in these instruments are perpendicular to one another. As discussed hereinafter, this design has disadvantages that are solved by the method of the present invention and the instruments that use that method.
Particles less than 1 .mu.m in diameter comprise more than 98% of the aerosol population in the atmosphere. Current on-line chemical characterization instruments that use light scattering to detect single particles in-flight are suitable for field studies but cannot detect ultrafine aerosols (i.e., particles with a diameter of less than about 200 nm). An example of this type of instrument is described in U.S. Pat. No. 4,383,171, which issued on May 10, 1983, to Sinha et al.
A solution to this problem was reported by W. D. Reents, Jr., et al. in an article entitled, "Single Particle Characterization by Time-of-Flight Mass Spectrometry", Aerosol Science and Technology 23: 263-270 (1995). The entire disclosure of this article is expressly incorporated herein by reference. In the Reents device, a focused pulsed excimer laser is used to ablate and ionize individual particles without the use of light scattering to sense the presence of a particle, followed by time-of-flight mass spectrometric analysis. By removing the light scattering element of the instrument, Reents was able to detect single particles as small as 0.02 .mu.m or 20 nm in diameter. However, the fraction of particles that were detected in the particle beam was unsatisfactory.